Antistatic property is very important in clean rooms. Static electricity causes contamination by dusts and failure and deterioration of electronic devices due to ESD (electrostatic dissipative) discharge. When charged dusts floating in the air approach a charged object, they are attracted by the object due to electrostatic induction and contaminate it. When static electricity charged in the human body or an object is discharged to an electronic device, discharge current passes through the low-resistance region of the device (see Table 1). Heat energy resulting from the current causes such problems as thermal failure of microchips. Particularly, although an operator does not sense the discharge voltage when it is 3.5 kV or below, an electronic device may suffer a severe damage.
As semiconductor products are becoming faster and less power-consuming, wirings and metal electrodes are becoming ever smaller and oxide insulator films are becoming thinner, for the purpose of circuit integration. Resultantly, even heat produced by a small electrostatic discharge current can lead to electrostatic failure of circuits. When a semiconductor product has been completely failed, it may be detected before it is shipped. But, a product slightly deteriorated by relatively small static electricity is hardly detected during shipment inspection.
In order to prevent these severe problems caused by static electricity, a variety of measures are taken, including earthing machines, apparatuses and equipments susceptible to static electricity, using antistatic-treated materials, using conductive materials, humidifying and installing static eliminator.
Especially, an operator wears an antistatic suite, antistatic shoes, antistatic gloves, etc. in order to minimize static electricity caused by the operator. Antistatic gloves are particularly important because they directly contact devices. Wearing antistatic gloves helps prevent contamination of devices and reduce electrostatic failure. Natural rubbers, synthetic rubbers (NBR), polyvinyl chlorides (PVC), etc. are currently used as materials for antistatic gloves. Natural rubber gloves have very poor antistatic property and tend to cause allergic reactions. Synthetic rubbers (NBR) also have insufficient antistatic property and are not completely free from allergic reactions. Polyvinyl gloves may cause contamination of devices because of migration of plasticizers and is disadvantageous in overall properties, including poor antistatic property, mechanical property and wearing comfort. Thus, gloves capable of offering superior antistatic property, wearing comfort, prevention of allergic reactions, mechanical property, etc., compared with the existing gloves, are needed.
TABLE 1Electrostatic sensitivity of electronic devicesESD voltage range causing failureDevice typeVoltage (V)Energy (μJ)VMOS 30-1800MOSFET100-200 0.5-2.0 EPROM1000.5JFET140-17000.98-2.45 OP-AMP190-25001.62-312.5 CMOS250-30003.13-450  Schottky diode300-25004.5-312.5Film register300-30004.5-450  Bipolar transistor100-70004.5-2.45 Schottky TTL1000-2500  50-312.5
Table 2 shows electric property based on surface resistance values according to ASTM D257. If the surface resistance is 1012 Ω/sq or larger, static electricity tends to form easily because charging is difficult. If the surface resistance is too small, spark may occur. When the surface resistance is 1010 Ω/sq or smaller, static electricity built up in a body decays almost instantly. When it is about 1012 Ω/sq, it takes a few seconds for the static electricity to decay, and several minutes at 1014 Ω/sq. Consequently, the best surface resistance range is from 105 Ω/sq to 1010 Ω/sq.
TABLE 2Electric property and surface resistanceClassificationSurface resistance (in ohms/square)Conducting<1 × 103 Chargeable<1 × 1012Insulating>1 × 1012
The present inventors succeeded in manufacturing antistatic gloves using a polyurethane showing physical properties comparable to those of a natural rubber and offering superior antistatic property. Manufacture of polyurethane gloves was disclosed in WO 00/61651 and WO 01/55237. In these patents, polyurethane was synthesized from a polyether polyol by forced emulsification using an external emulsifier, without introducing an ionic group inside the polyurethane. The resultant polyether type urethane gloves have poor hydrophilicity compared with the polyester type. Because an ionic group, which offers hydrophilicity, was not introduced, it has high surface resistance and poor antistatic property, as well as weak hydrophilicity. Although an external emulsifier was introduced to improve hydrophilicity, it is easily washed away by water, differently from the ionic group introduced inside the molecule. Resultantly, antistatic property of the film surface is significantly reduced after washing.
WO 01/88024 disclosed an aqueous polyurethane emulsion prepared by introducing a salt ion, a conductivity inducing material, into a polyether polyol by forced emulsification, in order to improve antistatic property. However, a film prepared using the same does not offer sufficient antistatic property.